In the field of forming duplicates in the graphic arts, it is required to increase work efficiency for coping with variety and complexity of printed matter.
Particularly in the lay-out process and in the contact process, work efficiency is improved by carrying out work under a more well-lightened environment. With this aim, presensitized plates capable of being handled under bright light and exposing printers have been developed.
The terminology "bright room-type photosensitive material" as used herein refers to photosensitive materials which can be used for a long period of time (not less than 5 minutes) under safelight (200 lux) not having a wavelength in the ultraviolet portion but consisting substantially of a wavelength of 400 nm or longer without substantial changes in photographic properties such that the 50% dot image can be reproduced with the change in dot areas of not more than 2% and the increase in fog of not more than 0.02.
The bright room-type photosensitive material to be employed in the lay-out process and contact process is a material for effecting negative-positive conversion or positive-positive conversion by using as an original a development-processed film having letter images or dot images and subjecting the original and the material to contact exposure (hereafter referred to as "contact-type photographic material"). Such a bright room-type photosensitive material is required to have (1) the property of effecting negative-positive conversion or positive-positive conversion of dot image, line image, and letter image in accordance with individual dot areas, line widths, and letter line widths of the original, and (2) the property of permitting the tone control of dot image, and line width control of line image and letter image. So far, bright room- and contact-type photosensitive materials capable of meeting such requirements have been provided.
However, in sophisticated image-conversion work of forming superimposed letter images through a superposed contact process, the conventional method of using a bright room-type photosensitive material and carrying out the contact process in a bright room has been found to have a defect of providing superimposed letter images inferior in quality to those provided by the method of using a conventional darkroom- and contact-type photosensitive material and carrying out the contact process in a bright room.
The method for forming superimposed letter images through the contact process is described in more detail below.
As shown in FIG. 1, letter or line image-forming film (line original) (b) and dot image-formed film (dot original) (d) are laminated on transparent or translucent bases (a) and (c), respectively, and the laminated bases are superposed to be used as an original wherein a polyethylene terephthalate film having a thickness of about 100 .mu.m is generally used as the bases. Dot original (d) is brought into direct contact with an emulsion surface of contact-type photosensitive material (e), and subjected to light exposure. The contact-type photosensitive material is then developed to produce white areas corresponding to line image in dot image.
In such a method for forming a superimposed letter image, it is highly desired that negative-positive conversion or positive-positive conversion be effected in accordance with individual dot areas of a dot original and individual line widths of a line original, respectively. However, as is apparent from FIG. 1, exposure for printing of line original (b) on contact-type photosensitive material (e) is carried out through base (c) and dot original (d), while dot original (d) is printed by exposure in direct contact with the emulsion surface of contact-type photosensitive material (e).
Therefore, the optimum exposure amount for accomplishing faithful negative-positive conversion or positive-positive conversion with respect to dot original (d) results in out of focus for line original (b) due to base (c) and dot original (d) interposed as a spacer. As the result, narrowing of the line width of white-printed image corresponding to the line original takes place. This is the cause for deterioration in quality of the superimposed letter image. This phenomenon is largely affected by the kind of an exposure source used other than the photographic property of the contact-type photosensitive material.